Ceramic materials have manifold uses in technology. However, their applicability is often dependent upon whether suitable bonding methods are known. The bonding of ceramic parts to one another and to metals presents considerable difficulties because of the differences between the materials.
Of the methods for producing positive material bonding in ceramics, it is primarily the conventional hard and soft soldering of metallized ceramic parts that is used in practice. This method requires many method steps, so that it becomes complicated and expensive to produce bonding of this kind.
In order to make it possible to dispense with the premetallizing of the ceramic parts, active soldering methods are used. These methods do not usually differ from the conventional soldering process performed in a protective gas or in a vacuum. As solder, however, hard solder having alloy components of very reactive metals, such as titanium, zirconium and others, is used. In oxide ceramics, the molten active solder reacts, working from the contact surface out, by reducing the surface layer of the basic material, forming suboxides. In this process, free chemical valences are produced at the surface of the ceramic, which are capable of bonding with the solder and thereby effecting good wetting of the molten solder and firm adhesion of the solidified solder on the ceramic. Furthermore, active metal oxides are deposited in the surface layer of the ceramic.
In the journal "METALL" (Metal), volume 27, March 1973, No. 3, pp. 259-265, the soldering behavior of various hard solder alloys are examined with respect to the basic material of aluminum oxide, in terms of their flowing behavior, wetting and adhesion behavior, infiltration and tensile strength. The greatest tensile strength values were attained with the 49Cu/49Ti/2B solder, which must be used under a high vacuum. The tensile strength amounted to 25% of the strength of the basic material. This solder also has very good flow properties on aluminum oxide. Equally good strength was ascertained for the 70Ag/27Cu/3Ti and 96Ag/4CuO solders. However, the flow behavior and wetting behavior of these solders is not always satisfactory. The hard solder alloy 49Cu/49Ti/2Be is a brittle solder alloy. It is used exclusively in powdered form under a vacuum. By contrast, the alloy 70Ag/27Cu/3Ti is a ductile solder alloy, which can be rolled out into foils of solder having a thickness of 0.1 mm. The solder 96Ag/4CuO is also a ductile solder alloy, which is suited only for soldering in an oxidizing atmosphere. The flow behavior on aluminum oxide is relatively good. The strength values that are attained, however, are considerably below those for the 49Cu/49Ti/2Be solder.